docs: Convert ImageInput-unassociatedalpha examples from docs to tests

[pfranz]

Description

Convert error checking C++ and Python examples from the "imageinput" chapter into tests within the "docs-examples" testsuites (#3992).

I grabbed unpremut.tif from the output of another test.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[lgritz]

I think this looks good, but it looks like it has merge conflicts because other people have had PRs merged recently in the same areas of those files. Can you please rebase on the current main and resolve any conflicts?